1. Field of the Invention
The present invention relates to fully implantable hearing aid system, and more particularly to an electret microphone adapted for use in such fully implantable hearing aid systems, and how such an electret microphone or other type of microphone may be incorporated into the fully implantable hearing aid system.
2. Description of the Prior Art
Patent Cooperation Treaty ("PCT") patent application no. PCT/US96/15087 filed Sep. 19, 1996, entitled "Implantable Hearing Aid" ("the PCT Patent Application") describes a fully implantable hearing aid system which uses a very small implantable microactuator. The PCT Patent Application also discloses a Kynar.RTM. microphone which may be physically separated far enough from the implanted microactuator so that no feedback occurs. The fully implantable hearing aid system disclosed in the PCT Patent Application can operate for a period of five years on a set of batteries, and produce sound levels of 110 dB. The fully implantable hearing aid system described in the PCT Patent Applications is extremely compact, sturdy, rugged, and provides significant progress towards addressing problems with presently available hearing aids.
While the Kynar microphone disclosed in the PCT Patent Application enables an operable fully implantable hearing aid system, that system's performance may be improved through the use of a more sensitive electret microphone. U.S. Pat. Nos. 4,947,478 ("the '478 patent") and 5,015,225, a division of the '478 patent, disclose incorporating a conventional electret microphone into an outer ear canal unit 34 of a partially implantable hearing aid system. U.S. Pat. No. 5,408,534 entitled "Electret Microphone Assembly, and Method of Manufacture" discloses an improved structure and method for coupling a charge plate of the electret microphone used in a hearing aid to an input terminal of an impedance matching circuit or internal amplifier. One difficulty with using an electret microphone for a fully implantable hearing aid system not addressed by the patents identified above is that the microphone must be hermetically sealed to prevent electret de-polarization while simultaneously permitting sound waves to impinge upon the microphone.
Because the hearing aid system disclosed in the PCT Patent Application is fully implanted, it is presently estimated that after a five year interval of use the system's battery may likely need replacement which necessarily involves surgery. Another aspect of a fully implantable hearing aid system is ensuring reliable electrical interconnection of the system's microphone and microactuator to the system's signal-processing amplifier throughout a five year interval prior to battery replacement, and subsequently after the battery has been replaced.